In general, SIR systems perform a number of functions including acceleration sensing, signal processing and analysis, and deployment of one or more restraint devices such as frontal or side air bags or seat belt pretensioners in response to a sensed crash event. Typically, the acceleration signal is monitored to detect a potential crash event, and then integrated over the course of the crash event to determine if the crash event is sufficiently severe to warrant deployment of restraints.
In addition to detecting a potential crash event, the system must distinguish between acceleration signals caused by driving over a rough road, and by a crash event. In the former instance, deployment is not desired, while in the latter instance, deployment is desired if the crash is sufficiently severe, as described above. The term "rough road" as used in this context is rather broad, and may include driving over a curb or a beam lying in the roadway.
The problem in distinguishing between a rough road and a crash event is that the rough road may produce relatively strong acceleration signals, perhaps even higher in initial magnitude than a crash event. Since the signals attributable to rough road impacts tend to be relatively short in duration compared to a crash event, a known deployment control technique is to reset the crash analysis function any time the acceleration signal falls below a calibrated threshold for at least a predefined period of time. While this technique thereby effectively ignores acceleration signals due to rough road impacts, it may result in delayed deployment in concatenated crash events where the initial impact resembles a rough road impact. Ideally, the system should have reset criteria calibrated to reject rough road impacts, and yet have the ability to deploy the restraints should a severe crash event immediately follow the rough road event.